An optical interferometric sensor for sensing current, the measureand, is shown and described in U.S. Pat. No. 5,644,397, entitled, “Fiber Optic Interferometric Current and Magnetic Field Sensor,” issued to the applicant of the present invention, and is incorporated herein by reference. In general, fiber optic current sensors work on the principle of the Faraday effect. Current flowing in a wire induces a magnetic field which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as:I=HdLwhere I is the electrical current, H is the magnetic field, and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire, and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The rotation of the state of polarization of the light due to the presence of an electrical current is measured by injecting light with a well-defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, a phase shift, Δφ, between two counter-rotating circularly polarized optical beams is induced into the pair of such beams traveling in the loop around the current carrying conductor in the presence of a magnetic field caused by the current flowing through the conductor.
In U.S. Pat. No. 5,644,397, there shown is in-line or linear interferometric fiber optic sensor for measuring current and magnetic fields. As taught therein, a light wave or beam from a light source is split into a pair of light beams that travel along first and second principle eigen axes; a birefringence modulator is supplied by a waveform or waveforms to modulate the birefringent light beams; a quarter waveplate is set at 45 degrees to the principle axes of the fiber to convert the orthogonally linearly polarized pair of light beams to counter-rotating circularly polarized light prior to entering the sensing region. Upon reflection at the end of the fiber, the sense of rotation of the two light beams are reversed and the light waves travel back through the sensing region, converted back to a pair of linearly polarized light beams, and propagate toward a photodetector and impinge thereon. The two light beams or waves therefore undergo reciprocal paths and the same polarization evolution through the optical circuit.
The fiber optic sensors taught by Blake in the aforementioned patent overcame many disadvantages associated with conventional all fiber sensors. However, the sensor and sensing method still suffered from a particularly exacerbating problem that affects the accuracy of the sensor. That is, to have a very accurate measurement, the optical components, particularly the quarter waveplate, must be perfect and not be affected by external stresses such as temperature variations and mechanical disturbances. It is well recognized that perfect or nearly perfect quarter waveplate are difficult and very costly to manufacture to achieve accurate sensing required by certain applications.
Some of the aforesaid shortfalls or problems are overcome by another optical interferometric sensor as shown and described in U.S. Pat. No. 5,696,858, entitled, Fiber Optics Apparatus and Method for Accurate Current Sensing, issued to the same applicant as the present invention. The aforesaid patent is also herein incorporated by reference in part and its entirety. Shown in FIG. 1 (labeled prior art) is substantially FIG. 1 of the aforesaid patent, similar in structure to the previously mentioned U.S. Pat. No. 5,644,397.
The fiber optic current sensor based on the in-line interferometric configuration as shown and described in the aforementioned patents is ideally a two-beam interferometer. One beam travels down a polarization-maintaining (PM) fiber delay line in the x-polarization, and the other in the y-polarization. At the end of the PM fiber delay line, a quarter waveplate converts these two beams into RHCP and LHCP circular polarization states.
The quarter waveplate generally defines the beginning of the sensing region. The sensing fiber which follows—that part of the optical path where the pair of beams are affected by the measureand, ideally maintains the circular polarization states of the two beams. The two beams accordingly accumulate a phase shift in proportion to the magnetic field along the fiber. Generally, the sensing fiber is wound in multiple loops and terminates in a mirror located in close proximity to the spatial position of the quarter waveplate.
For a current sensor, the sensing fiber comprises a “closed path” around the current carrying conductor, and the total accumulated phase shift experienced between the two beams is related to the closed integral of the magnetic field around the conductor, which by Ampere's law is linearly related to the current carried by the conductor.
Continuing, the in-line interferometer is such that the pair of light beams swap circular states of polarization upon reflection from the mirror, and continue to accumulate a phase shift or difference as the pair of beams travel through the sensing fiber in the opposite direction but with opposite polarization states,—the beam that traveled through the sensing fiber as a RHCP beam returns as a LHCP beam.
Upon again reaching the quarter waveplate from the opposite direction, the two beams are returned to linear polarization states. However, the original x-polarized beam returns as a y-polarized light beam for the return trip. The returning two linear polarized light beams are “interfered” in the polarizer, and the interfered light is routed to impinge upon a photodetector that provides an output signal related to the sum of the pair of light beams impinging thereon. In turn, signal processing electronics responsive to the output of the photodetector provides an output signal indicative of the current flowing in the conductor or another selected measureand for differing applications.
In the ideal in-line fiber optic current sensor described above, the two beams interfere with perfect “visibility”, and the phase shift between them is linearly related to the current in the conductor passing through the sensing region. In a practical embodiment, a birefringence modulator is advantageously placed serially in the PM fiber delay line to modulate the phase difference between the two interfering beams to aid in the detection of the conductor current induced phase shift as taught in the aforementioned current sensor patents. However, errors arise in the in-line fiber optic current sensor when stray polarization coupling points exist in the optical circuit. These stray polarization coupling points can be due to imperfect splices, imperfect connectors, an imperfect quarter waveplate, or the like.
Light that has cross-coupled in the optical circuit exhibits itself in two important ways. First, some light that eventually participates in the interference has traveled through the sensing region in the wrong polarization state and picks up the wrong phase shift from the current flowing in the conductor or a particular measureand. This cross coupled light acts to alter the relationship (or “scale factor”) between the current flowing in the conductor and the phase shift interpreted to exist by the signal processing electronics. Second, some light travels an incoherent path with respect to the main interfering waves, and adds an “offset DC component” of light to the photo-detector.
It is important to note that the amount of offset DC light falling on the photodetector is related to the amount of light existing in the spurious coherent waves that shift the scale factor of the sensor. This principle has been noted in and described in the aforementioned U.S. Pat. No. 5,696,858, and was there used to show that by normalizing the sensor output to the peak intensity observed at the photodetector for a modulated system, the errors due to these cross-coupling effects can be reduced from second order to fourth order.
Although the just mentioned patent and solution provided marked improvement, certain sensor applications require consideration of the fourth order errors, and thus there is a need for a further improvement in the optical interferometric sensor of the prior art.